The present invention relates to a spark-ignition direct injection engine with supercharger, which has been provided with a supercharger for boosting intake air and which stratifies fuel that has been directly injected into the combustion chamber around the electrodes of the spark plugs within the cylinders and combusts it, and in particular relates to the technological field of combustion control in a specific driving region on the high-speed high-load side.
In a conventional example of this type of spark-ignition direct injection engine with supercharger disclosed in JP 2000-274278A, for example, the supercharger is stopped when the engine is in a state of stratified combustion, whereas flow within the cylinder is strengthened by boosting when the engine is in a state of homogenous combustion. This means that during low speeds and low loads at which a state of stratified combustion is assumed, the boosting of intake air by the supercharger is stopped or inhibited to leave a relatively weak state of flow within the cylinder, thereby keeping the air-fuel mixture from diffusing within the combustion chamber and thus achieving suitable stratification. On the other hand, during high-speeds and high-loads at which a state of homogenous combustion is assumed, the flow within the cylinders is strengthened by boosting to adequately mix the intake air with the large volume of fuel injected into the combustion chamber during the intake stroke and form an air-fuel mixture that is as uniform as possible.
In general, in an engine with a supercharger, only a portion of the exhaust is supplied to the supercharger when there is a high exhaust flow and/or pressure, and the rest is released into the air to keep the boost pressure of the intake air due to the supercharger below a target value (maximum boost pressure). This also applies to the above conventional example, where a portion of the exhaust bypasses the supercharger and flows down the exhaust pipe when the engine is in a high-speed high-load driving state. A portion of the energy held by the high pressure exhaust is discarded at this time.
However, due to the structure of a spark-ignition direct injection engine, which directly injects fuel into the combustion chamber within the cylinders of the engine, the period in a single combustion cycle during which fuel injection is possible is restricted to the intake stroke and the compression stroke of the cylinders. Thus, not enough time can be secured between fuel injection and ignition, and during high-load operation where a greater volume of fuel is injected, it is difficult to sufficiently vaporize and atomize the fuel before the cylinders are fired. As a consequence, a portion of the fuel is baked, and this leads to the discharge of particulate matter (hereinafter, referred to as PM) like in diesel engines.
A greater volume of fuel is injected particularly when the engine is in a high-speed high-load driving state, however, the time interval during which the fuel can be injected is shortened inversely proportional to the increase in engine revolution speed. This retards the end of the fuel injection, which further shortens the period between fuel injection and ignition and complicates vaporization and atomization. Also, there is a drop in the flow within the cylinder during the compression stroke of the cylinder compared to during the intake stroke, and thus during the compression stroke it becomes difficult to promote the fuel injected into the cylinder to mix with the air. This also hinders fuel vaporization and atomization.
Additionally, giving consideration to exhaust system reliability, in spark-ignition engines, the air-fuel ratio is generally controlled in a specific driving region on the high-speed or the high-load side so that it is richer than the theoretical air-fuel ratio in order to keep the exhaust temperature from rising due to the latent heat of excess fuel. Thus, even if the intake air is boosted by the supercharger to strengthen the flow within the cylinder when the engine is in this specific driving region, as in the above conventional example, the resulting effect is surpassed by the effect of the large volume of fuel that is supplied, making the above problem of PM even more conspicuous.
The present invention was arrived at in light of the above problems. It is an object of the present invention to exploit the fact that spark-ignition direct injection engines that are provided with a supercharger conventionally discard a portion of the exhaust energy during high-speed high-load engine operation, and a solution has been adopted to control, for example, the boost pressure or the like particularly in a specific driving region on the high-speed high-load side in order to ensure maximum engine output and exhaust system reliability while also achieving a reduction in PM within the exhaust.
To achieve the above object, one solution presented by the present invention is to collect the portion of exhaust energy that has conventionally been discarded in the specific driving region on the high-speed high-load side, and effectively use it to maximize the strength of the flow within the cylinder.
More specifically, as shown illustratively in FIG. 1, it is a premise of a first aspect of the present invention that a spark-ignition type 4-cycle direct injection engine 1 is provided with a supercharger 40 for boosting intake air to the cylinder and a fuel injection valve 18 for directly injecting and supplying fuel to a combustion chamber 6 within the cylinders, and that fuel is injected during the intake stroke of the cylinder 2 by the fuel injection valve 18 in at least a supercharge region on a high-speed high-load side to attain a state of homogenous combustion.
This configuration is also provided with a flow strengthening means 30 for constricting the flow of intake air into the cylinder in order to strengthen the flow within the cylinder, a boost pressure adjustment means 42 for keeping the boost pressure of the intake air due to the supercharger 40 below a target boost pressure, an air-fuel ratio control means 50b for controlling an air-fuel ratio A/F in the cylinder to become A/Fxe2x89xa613 in a specific driving region that is established on the high-speed high-load side of the supercharge region, a flow control means 50c for increasing the amount of constriction on the intake air caused with the flow strengthening means 30 so as to relatively strengthen the flow in the cylinder in the specific driving region as compared to that in a region that is adjacent to the low-load side of the specific driving region, even if the engine revolution speed is the same, and a boost pressure control means 50d for controlling the boost pressure adjustment means 42 to relatively increase the target boost pressure in the specific driving region as compared to that in a region that is adjacent to a low-speed side of the specific driving region.
According to this configuration, first, when the engine 1 is in a predetermined high-speed high-load driving region (specific driving region), the air-fuel ratio within the cylinder 2 of the engine 1 is enriched by the air-fuel ratio control means 50b to become A/Fxe2x89xa613, and the temperature of the exhaust is kept from rising due to the latent heat of the overly large volume of fuel compared to the volume of air. Also, the boost pressure adjustment means 42 is controlled by the boost pressure control means 50d to raise the target boost pressure, and the flow strengthening means 30 is controlled by the flow control means 50c to increase the amount of constriction on the intake air, so that the combination of these actions strengthens the flow within the cylinder as much as possible and sufficiently promotes fuel vaporization and atomization.
That is, in the specific driving region on the high-speed high-load side, the exhaust energy that has conventionally been discarded is utilized to further boost the intake air with the supercharger 40, and this intake air can then be constricted to maximize the strength of the flow in the cylinders. A result of this is that a large volume of injected fuel can be sufficiently vaporized and atomized, the PM within the exhaust can be reduced, and a rise in the exhaust temperature due to the latent heat of the large volume of fuel can be effectively inhibited. The drop in intake efficiency as the intake air is constricted is compensated by the rise in boost pressure, and therefore that the maximum output of the engine 1 does not drop.
According to the engine of a second aspect of the present invention, the boost pressure control means increases the target boost pressure in order to compensate the drop in the intake efficiency that is caused when the flow control means controls the flow strengthening means to increase the amount of constriction on the intake air.
Thus, even when the engine is in the high-speed high-load specific region and the amount of constriction on the intake air by the flow strengthening means has been increased, which causes the intake efficiency of the cylinders to drop, the boost pressure is increased accordingly to compensate the drop in intake efficiency of the cylinders. Consequently, maximum engine output can be reliably maintained and fluctuations in the engine output are kept from occurring when the driving state of the engine shifts between the specific driving region and other driving regions.
According to the engine of a third aspect of the present invention, the flow control means is configured so that it minimizes the amount of constriction on the intake air due to the flow strengthening means in at least the supercharge region aside from the specific driving region.
Thus, in at least the supercharge region aside from the specific driving region, the drop in intake efficiency that accompanies the constriction of the intake air is minimized, and an improvement in fuel efficiency is achieved by the reduction in pumping loss. It should be noted that sufficient flow within the cylinders due to boosting can be obtained even if a special effort is not made to constrict the intake air outside the specific driving region, and thus PM discharge does not become a problem.
According to the engine of a fourth aspect of the present invention, the flow strengthening means is provided with a flow control valve that is disposed in an intake passage to the cylinder and with an actuator for adjusting the opening degree of the flow control valve, and the flow control means is configured so that it controls the opening degree of the flow control valve by the operation of the actuator.
Thus, the flow strengthening means is given a specific configuration, and the flow control valve is closed by the flow control means to constrict the intake air and thereby reliably strengthen the flow within the cylinders.
According to a fifth aspect of the invention, in the engine according to the fourth aspect of the invention, the opening degree of the flow control valve is reduced to strengthen the tumble flow, which serves as the flow in the cylinder, and the fuel injection valve is disposed in opposition to the tumble flow that flows through the combustion chamber in the cylinder during the compression stroke of the cylinder. Also, a fuel injection control means is provided, which in a predetermined driving region on the low-speed low-load side, injects fuel toward the tumble flow during the compression stroke of the cylinder through the fuel injection valve, so that the fuel injected by the fuel injection valve becomes a combustible air-fuel mixture in the spark period of the cylinder and stays near the electrode of the spark plug, and the flow control means is configured so that it closes the flow control valve in the predetermined driving region and in the specific driving region.
According to this configuration, when the engine is in a predetermined driving region on the low-speed low-load side, the flow control valve is closed by the flow control means in order to strengthen the tumble flow within the cylinder, and the fuel that is injected toward this tumble flow at a predetermined timing is slowed by the tumble flow and stratified around the electrode of the spark plug in the spark period of the cylinder. That is, in the low-speed region of the engine, in which there is low air intake speed, the tumble flow has originally been strengthened to balance it with the penetration of the fuel spray in order to attain suitable stratification of the air-fuel mixture.
However, when the engine is in the specific driving region on the high-speed high-load side, the tumble flow within the cylinder can be reliably strengthened by closing the flow control valve using the flow control means. That is, the flow control valve that is necessary in achieving a suitable stratified combustion at times of low-speed and low-load can also be effectively utilized at times of high-speed and high-load so that the effect of the fourth aspect of the invention can be adequately achieved without an increase in costs or a complication of the structure.
According to a sixth aspect of the present invention, a spark-ignition type 4-cycle direct injection engine is provided with a fuel injection valve for directly injecting and supplying fuel to a combustion chamber within a cylinder, a turbocharger for receiving exhaust from the combustion chamber and boosting intake air, and a controller for injecting fuel with the fuel injection valve during the intake stroke of the cylinder so as to attain a state of homogenous combustion in at least a supercharge region on the high-speed high-load side.
Moreover, it is also provided with a flow control valve that is disposed in an intake passage to the cylinder and with a relief valve for bypassing a portion of the exhaust from the combustion chamber to the downstream side of the turbocharger to keep the boost pressure of the intake air below the target boost pressure. Also, the controller is configured so that, in a specific driving region established on the high-speed high-load side within the supercharge region, it controls the volume of fuel injected by the fuel injection valve so that the air-fuel ratio A/F in the cylinder becomes A/Fxe2x89xa613, and also controls the opening degree of the flow control valve in order to relatively strengthen the flow in the cylinder as compared to that in a driving region adjacent to the low-load side of the specific driving region, even if the engine revolution speed is the same, and controls the opening degree of the relief valve in order to relatively increase the target boost pressure as compared to that in a driving region adjacent to the low-speed side of the specific driving region.
This configuration achieves the same effect as the first aspect of the invention.
In an engine according to a seventh aspect of the invention, in the engine according to the sixth aspect of the invention, the controller is configured to increase the target boost pressure by controlling the opening degree of the relief valve and thereby compensate the drop in intake efficiency that is caused when the amount of constriction on the intake air is increased by the flow control valve of the intake passage.
This configuration achieves the same effect as the second aspect of the invention.
As explained hereinabove, according to the spark-ignition direct injection engine with supercharger of the first aspect of the present invention, when fuel is injected during the intake stroke of the cylinder by the fuel injection valve to achieve a state of homogenous combustion in at least the supercharge region on the high-speed high-load side, the air-fuel ratio in the cylinder of the engine is enriched by the air-fuel ratio control means in a specific driving region in the supercharge region to inhibit an excessive rise in the exhaust temperature, while the intake air is further boosted by the supercharger and constricted to strengthen the flow within the cylinder to a maximum level and thus ensure maximum engine output while adequately vaporizing and atomizing the large amount of fuel, in order to reduce PM within the exhaust.
According to the second aspect of the invention, by increasing the boost pressure in the specific driving region in order to compensate the drop in intake efficiency due to the constriction of the intake air, it is possible to maintain maximum engine output and inhibit fluctuations in engine output.
According to the third aspect of the invention, by minimizing the amount of constriction on the intake air in at least the supercharge region aside from the specific driving region, it is possible to reduce pumping loss in the supercharge region and improve fuel consumption efficiency.
According to the fourth aspect of the invention, it is possible to reliably strengthen the flow within the cylinder by closing the flow control valve disposed in the intake passage to the cylinder to constrict the intake air.
According to a fifth aspect of the invention, in a so-called air guide direct injection engine, which requires a flow control valve in order to strengthen the tumble flow during stratified combustion, the flow control valve can be effectively utilized to adequately attain the effect of the fourth embodiment without an accompanied increase in costs or a complication of the structure.
Also, according to the sixth and seventh aspects of the invention, the same effects as in the first and second aspects of the invention, respectively, can be obtained.